first thing I would try would be an exception to SSL Scanner. If that works it is possible that the store validates the server certificate, which is replaced by MWG if SSL Scanner is turned on. If that does not help some more research would be required.
I had actually read that on other sites as well, but was no able to install policy untill yesterday. There were three or four sites that I had to exempt from SSL scanning in order for the store to install apps.
FWIW can you please provide the URLs that you exempted from SSL Scanning?
I am experiencing the same issue.